A critical function of Emergency Core Cooling Systems (ECCS) and other recirculation systems of nuclear power plants is to move fluids quickly and in large volumes to critical areas of the nuclear power plant in the event of accidents and emergencies. Integral to this critical function is the ability of strainers, filters, screens and other such devices associated with the systems to remove solids from the moving fluids while at the same time maintaining a sufficiently large volume of fluid flow.
Nuclear plants have various safety systems to ensure that the nuclear fuel in the reactor core remains cooled in all credible accident scenarios. One such scenario is a “loss of coolant accident,” (LOCA) in which an external pipe is postulated to break, allowing a large amount of water to escape from the reactor cooling system. This water may dislodge solid debris from neighbouring pipes or other reactor structures. The water, along with some of the dislodged debris, will flow to the lowest parts of the reactor building into a sump. Plants are equipped with safety systems that pump water from the sump back into various reactor cooling systems. Strainers on the pump intakes ensure that any debris large enough to clog equipment in these systems is prevented from entering.
Depending on the type of debris, the first layer to deposit on the strainer may form a mat of fibers and collect finer particles, which would otherwise pass through the strainer, resulting in a thin layer of low porosity debris with high hydraulic resistance. This behaviour is referred to as the “thin-bed effect” where the head loss per unit thickness of debris is relatively high as compared to that of full (or thick-bed) debris formation where relatively high porosity debris allows the passage of flow with lower head losses. Thin-bed debris can cause head losses high enough to threaten the functionality of emergency core cooling system (ECCS) sump recirculation pumps. Thin-bed debris has occurred operationally at nuclear power plants and has been created during head-loss testing. One way of alleviating the thin-bend effect is to increase the surface area of ECCS strainers.
Strainers must have enough screen area that the debris layer on the strainer is not too thick to cause unacceptably high restriction to flow. Strainers must also be as small as possible to fit into the available space. Therefore compactness, i.e., accommodating the most screen area in the smallest volume, is important.
Conventional strainers in many nuclear plants are simple box-type devices that were mounted over the pump intakes. Newer more advanced strainers often have an irregular surface to increase the surface area. An example of an advanced strainer is Atomic Energy of Canada Limited's (AECL's) Finned Strainer®, which is described in International PCT publication number WO 06/50606. The Finned Strainer performs the filtering function through modular hollow fins attached to a header that directs the filtered water to pump intake. The Finned Strainer includes two different fin designs (1) Flat-Surface Fins and (2) Corrugated Surface Fins. These fins have porous filtering surfaces.
There remains a need for an improved strainer or filtering element, for example one that minimizes the thin-bed effect described above, and which can be incorporated into existing systems.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.